What were the Ace science fiction double novels? Andrew Liptak explains:
From the start, Ace began an innovative approach to their novels by printing them Tête-bêche style. Each volume contained two short novels, with a book on each side, flipped 180 degrees from the other. A reader would pick up one book, read through it, and flip the book to read the other…..
So these books were all over the place, beckoning to the young science fiction addict. In drug store paperback racks, everywhere. And at book sales, where I discovered a stash of them at my local elementary school book sale. What a happy day that was! One of the ones I grabbed is the double novel pictured above. I must have had at least 8 or 10 other ones. Anyway, these double novels were a very important part of the American science fiction landscape in the fifties and sixties and can still provide much retro delight if one chooses carefully. Plus they're just a lot of fun to look at (quite collectible too, I understand).In October 1953, Ace introduced its first science-fiction novel, pairing up two novels from A.E. van Vogt: The World of Null-A and The Universe Maker. Every other month, a new book would appear on the book rack. Van Vogt’s book was followed in December 1953 by Robert E. Howard’s Conan the Conqueror and Leigh Brackett’s The Sword of Rhiannon. In between, the company published a single novel. Due to their popularity, Ace bumped the production schedule up to a science-fiction double a month in 1958.
Some reviews of Ace double novels can be found here and here. Wikipedia also has a complete chronological list of all titles in the series. So fire up that Wayback machine and enjoy.